1. Field of the Invention
The present invention relates to a shimmed magnetic resonance imaging apparatus and method for shimming magnetic resonant equipment, particularly a shimming method for an irregular object to be examined by magnetic resonant equipment.
2. Description of the Prior Art
In magnetic resonant equipment, the degree of magnetic field homogeneity has direct influence on the image quality. Within a scan region, the larger the homogeneous region, the better the image quality. Conventionally, in order to achieve a larger homogeneous region, the size, complexity and weight of magnetic resonant equipment (scanners) have been increased dramatically, leading to a significant increase in costs.
At the same time, due to the above reason, the size increase of the magnetic resonant equipment leads to reduced openness, therefore to restriction as to the applications for such scanners.
For an open and flat magnet, such as a C-shaped magnet, in order to reduce the effect of claustrophobia, the open distance (spacing) should be designed as large as possible. In such magnetic resonant equipment, when a patient lies down on a scanning bed, the distance from the patient to the upper magnetic pole is far more than the distance to the lower magnetic pole so as to improve the openness effect. In other words, the patient is not in the middle between the two magnetic poles but is closer to the lower magnetic pole.
Although during the design stage the magnets are made as homogeneous as possible, the magnetic field homogeneity produced by the manufactured magnets is usually not as good as expected, so the final magnetic field homogeneity has to be achieved by shimming, such as by adding soft magnetic or hard magnetic shims onto the magnetic poles. In order to correct the spatial inhomogeneity in a magnetic field, the shim size should be as small as possible. Small shims, however, generate in their vicinity a spatial inhomogeneity of higher powers, which makes it very difficult to perform shimming close to a magnetic pole.
For a hollow, cylindrical and horizontal magnet, in order to reduce the effect of claustrophobia, a patient usually lies in a bed closer to the lower side of the hollow channel, so as to increase the patient's distance to the upper side of the hollow channel. The magnetic field homogeneity of the manufactured magnet again is not as homogeneous as by theoretical calculation, so the final homogeneity also has to be achieved by shimming, such as by inserting shims into pre-made grooves on the periphery of the hollow channel.
For flat magnets, the homogeneous region is usually designed at a position that is symmetrical relative to the upper and lower magnetic poles, but for a hollow cylindrical magnet, the homogeneous region is designed at a position symmetrical relative to the cylindrical axis. In the prior art, the following methods are usually used to improve magnetic field homogeneity in magnetic resonance imaging systems:
(1) The magnets are produced with a more optimized design, for example by optimizing the source for generating the magnetic field and the magnetic circuit so as to achieve a better and bigger homogeneous region, and this method is intended to improve the overall homogeneity of the magnetic field;
(2) Shimming is performed for an ellipsoidal volume instead of a spherical volume, which leads to reduced shimming requirements in the short axis direction of the ellipsoid;
(3) The requirements for the field homogeneity are reduced by designing different imaging sequences.
Referring to FIG. 1, in the prior art the homogeneous region of the magnetic field is usually a spherical homogeneous region 300 or an ellipsoidal homogeneous region 400. Because a patient to be scanned has the shape of an irregular object to be examined 200, and as mentioned above, the patient is usually positioned closer to the lower magnetic pole and farther away from the upper magnetic pole, therefore the patient is not located at the middle position of the homogeneous region formed between the upper and lower magnetic poles. In order to cover the area below the irregular object to be examined 200, the volume of this spherical shaped homogeneous region 300 or the ellipsoidal shaped homogeneous region 400 has to be big enough, which means a great increase of the homogeneous region that is not within the image scanning area and that has no contribution to the image quality.
Therefore, in the above schemes, the magnet design optimization improves the magnetic field's overall homogeneity to a certain degree, but cannot solve the problem of creating redundant homogeneous regions having no contribution to image quality. The use of an ellipsoidal shaped field region instead of a spherical shaped field region for shimming can reduce the homogeneous regions having no contribution to image quality to a certain degree, but this still is not sufficient when scanning an irregular object to be examined that has a more complicated shape. Designing different imaging sequences to reduce requirements to field homogeneity can reduce equipment costs, but nevertheless cannot solve the problem of non-contributing homogeneous regions, and the use of different imaging sequences leads to long acquisition times and slow image formation speed.